1. Field of the Invention
The present invention relates to an apparatus and method for controlling the deposition of a powder in a plasma spray process, and particularly to an apparatus and method in which the location and pattern of powder deposition is monitored and controlled.
2. Description of Related Art
Heretofore, problems have existed in powder deposition plasma spray processes in that it is very difficult to control the precise location of the powder deposit. Processes employing low pressure dc-arc plasma spray guns generally incorporate a cross-flow powder injection scheme which, along with variations in powder size and flow swirl, contributes to the inconsistencies in depositing the powder in a desired location. In a device employing cross-flow powder injection, the powder is delivered to the plasma gun by a carrier gas which serves two purposes. In conveying the powder from the powder feeders to the gun, the gas must flow at a high enough rate to ensure that the powder does not settle and plug the powder lines. In addition, once the gas reaches the powder injector, the gas is required to accelerate the powder preferably to a desired speed at which the powder will penetrate the plasma jet to its central hot region and then be melted and deposited on the target. If the gas flow rate at the injector is too high, the powder will completely traverse the plasma jet and will not be completely melted, and if the gas flow rate is too low, the powder will not penetrate into the hot core of the jet. Difficulties previously encountered with cross-flow powder injection are believed to be attributable to a mismatch between the carrier gas flow rate required to ensure a free flow of powder through the lines and the gas flow rate required to properly inject the powder into the plasma jet.
A further problem has existed with the cross-flow powder injection devices previously employed. Erosion of the feed port causes variations in the injection speed and trajectory of the powder. The solution presently employed to correct this problem is to change the guns after approximately 100 hours of running time. The new gun must then be adjusted to ensure that the spray pattern falls within certain limitations before continuing with the process. Prior to the present invention, no system or device was believed to exist which provided a means for automatic on-line compensation of the spray pattern, which would reduce the amount of adjustment necessary to obtain the desired spray pattern.
In devices and processes employing an RF plasma spray gun, an axial powder feed is employed, which avoids several of the above-noted problems associated with the cross-flow feeding of the powder. However, problems in controlling the deposit location or pattern exist in systems employing RF guns, in that RF gun deposits "wander" on the target due to complex flow patterns within the guns. The location of the deposit on the target in such systems is dependent upon the degree of injector insertion into the gun and plasma. Prior to the present invention, no system or device was believed to exist which was used to monitor the deposit to locate the impact point of the powder on the target, and to use the information obtained in a feedback loop to modulate the injector insertion.
U.S. Pat. No. 4,656,331, issued to Lillquist et al, and assigned to General Electric Company, discloses an infrared sensor suitable for use in detecting temperatures of particles entrained in a plasma spray jet in order to control the electrical power input to the plasma torch to ensure that the particles are heated to a molten temperature prior to their impact on a target substrate. The infrared sensor disclosed in that patent is discussed as having, alternatively, a single detector, a linear array of detectors to measure a temperature profile or beam divergence, or a rectangular array of detectors capable of performing an imaging function.
It is a primary object of the present invention to provide an apparatus for detecting and monitoring the deposition of the powder on the target, and providing means for using information obtained to selectively adjust, as necessary, one or more parameters in order to control the deposition of the powder on the target.
It is an additional object of the present invention to provide an apparatus having an infrared imaging radiometer integrated with a video signal processor for providing information related to the sensed location of the deposit of the powder onto the target.
It is an additional object of the present invention to provide an apparatus which provides improved and more accurate delivery of the powder into the plasma jet by compensating for variations in powder size and/or by adjusting the carrier gas flow rate in the powder injector.
It is an additional object of the present invention to provide a method for controlling the deposition of a powder on a target using a plasma spray process wherein a pattern of powder deposition on the target is monitored by an infrared imaging radiometer to determine the impact point of the powder, a display of the imaged pattern is produced, and one or more parameters, including the carrier gas flow rate, are adjusted as necessary to more the powder impact point to yield the desired deposition pattern.
It is a further object of the present invention to provide an apparatus having means for controlling the flow rate of the carrier gas in the powder injector such that an optimal flow rate may be achieved in both the powder supply lines and in the powder injector tube.